Project Summary Typhoid fever and paratyphoid fever are severe systemic infections caused by strictly human-adapted Salmonella enterica serovar Typhi (STY) and S. enterica serovar Paratyphi A (SPA), respectively. The clinical presentation of paratyphoid fever is indistinguishable from typhoid fever but differs markedly from that of human gastroenteritis caused by non-typhoidal Salmonella serovars. Differences in the clinical presentation of typhoid fever and gastroenteritis have been attributed to an evasion of innate immune responses mediated by STY-specific virulence factors Salmonella pathogenicity island 7 (SPI7) that encodes biosynthesis genes for the production of a capsular polysaccharide which prevents complement activation, and a regulatory protein that inhibits invasion gene expression after entry into the epithelial mucosa, thereby reducing inflammatory responses. SPA pathogenesis is not well studied but SPI7 is not present in this pathogen, which is puzzling given the similarities in the clinical presentation of typhoid and paratyphoid fever. Here we propose to address this paradox, thereby filling a key gap in knowledge about the pathogenesis of paratyphoid fever. Our central hypothesis is that SPA obstructs complement activation using modifications in its O-antigen and suppresses invasion gene expression when it encounters increased oxygen availability upon epithelial invasion. We will determine how the O-antigen enables SPA to evade complement activation Aim 1), and we will identify genes responsible for aerobic control of SPA invasion gene expression (Aim 2). The proposed work is innovative because it will usher in new concepts that help explain why the clinical presentation of infections with typhoidal Salmonella serovars differs dramatically from gastroenteritis caused by non-typhoidal Salmonella serovars. It is our expectation that successful completion of the proposed experiments will establish the concept that inhibition of complement activation and suppression of invasion gene regulation upon entry into the epithelium are features of a pathogenic strategy that distinguishes typhoidal Salmonella serovars from those associated with human gastroenteritis.